How? To use a scenario that HNers are likely more familiar with: this is like when a video game company does preorders for a video game. Or a concert promoter

How? To use a scenario that HNers are likely more familiar with: this is like when a video game company does preorders for a video game. Or a concert promoter sells tickets before the event is built, etc. The purpose is to offset risk. Pfizer didn't have to take a risk on this vaccine, because they had already sold enough of it to cover their development costs. They may have "used their own money", but that ultimately comes down to an accounting technicality for a company that large.